The present invention relates to an ignition coil for generating a high voltage that is applied to spark plugs of an internal combustion engine and a method for fabricating the same ignition coil.
An internal combustion engine ignition coil (hereinafter, simply referred to as an xe2x80x9cignition coilxe2x80x9d) is a device for generating a spark across a gap of a spark plug by producing a high voltage through mutual induction actions of coils. There are several types of ignition coils. For example, there is a stick-type ignition coil adapted to be installed in a plug hole and this stick-type ignition coil has a rod-like core, a cylindrical secondary spool disposed around the outer circumference of the core, a secondary coil wound around the secondary spool, a cylindrical primary spool disposed around the outer circumference of the secondary coil and a primary coil wound around the primary spool. Namely, the core, secondary spool, secondary coil, primary spool and primary coil are disposed coaxially in that order from the inner circumference of the ignition coil. These members are accommodated in a hollow cylindrical case. In addition, in order to secure electric insulation between the respective members and to allow the members to adhere to each other in the case, a resin insulating material is filled in the case.
In this respect, a base resin constituting, in particular, the spool of the primary and secondary spools which is disposed between the primary coil and the secondary coil (the primary spool in the aforesaid conventional example) has conventionally been required to have high electric insulation. This is because, in the case where a failure of insulation occurs to allow the secondary coil side, that is, the high-tension side and the primary coil side, that is, the low-tension side to electrically communicate with each other, there is a risk that a desired voltage cannot be secured on the secondary coil side.
In addition, the base resin constituting, in particular, the spool of the primary and secondary spools which is disposed between the primary coil and the secondary coil has conventionally been required to have a high adhesion to the resin insulating material. This is because the coefficient of linear expansion of the base resin of the spool is different from that of a wire constituting the coil which is wound around the spool, and, due to this, if the adhesion between the resin insulating material filled between the spool and the wire and the base resin of the spool is low, there is a risk that the spool and the resin insulating material may separate from each due to thermal stress. If the spool separates from the resin insulating material, a corona discharge is produced within a space formed by the separation, leading to a risk that electric insulation between the primary coil and the secondary coil cannot be secured.
Thus, the base resin constituting the spool has conventionally been required to be highly insulating and to have high adhesion to the resin insulating material.
In order to satisfy the aforesaid requirements, conventionally used for the base resin of the spool have been polyphenylene ether (PPE), polybutylene terephthalate (PBT), polyethylene terephthalate and the like which are highly insulating and have high adhesion to the resin insulating material.
However, when the spool is formed of a base resin which has a high adhesion to the resin insulating material, the following problems occur. Namely, as the coefficient of linear expansion of the base resin is different from that of the wire constituting the coil, if the ignition coil is used under a thermal cycling environment where the temperature is raised and lowered repeatedly, thermal stress is produced repeatedly in the spool due to the difference in coefficient of linear expansion. This thermal stress so produced can be relaxed only if the spool separates from the resin insulating material. However, the adhesion between the spool and the resin insulating material is made high in order to restrain the separation. Due to this, the thermal stress cannot be relaxed as desired, and there may be incurred the risk that cracks are produced in the spool. Then, if cracks are produced in the spool, the high-tension side and the low-tension side are allowed to electrically communicate with each other, leading to a risk that the desired voltage cannot be secured.
To cope with this, for example, with a conventional ignition coil disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-111545, as shown in FIGS. 9A, 9B, a separation tape 203 was wound between a spool 200 and a resin insulating material 202 filled on a wire 201 side. The generation of thermal stress attributed to the difference in coefficient of linear expansion between the spool 200 and the resin insulating material 202 in FIG. 9A and between the spools 200 and 206 and the wire 201 and resin insulating materials 202, 204 in FIG. 9B was restrained by separating the spool 200 from the resin insulating material 202 with the separation tape 203, whereby the generation of cracks in the spools 200 and 206 was restrained.
In addition, with the conventional ignition coil, in order to restrain the generation of cracks in the spools, a rubber component such as styrene ethylene butene styrene (SEBS) was added to a base resin for the spools. Then, the toughness of the spools was enhanced by the rubber component so added to thereby restrain the generation of cracks in the spools.
Thus, with the conventional ignition coil, in order to suppress the generation of cracks in the spool, a separation tape was wound around the spool or the rubber component was added in the spool, which served not only to increase the production costs of the ignition coil but also to complicate the production process.
Incidentally, the aforesaid problems are attributed to the high adhesion between the base resin constituting the spool and the resin insulating material. To cope with this, if a resin such as polyphenylene sulfide (PPS) introduced in Japanese Unexamined Patent Publication (Kokai) No. 8-339928, which has a low adhesion to the resin insulating material, is used as the base resin, the risk that cracks are produced in the spool will be reduced.
However, when compared with PPE, PBT, and PET, PPS has lower electric insulating properties. Due to this, if PPS is used as the base resin, due to the low adhesion inherent in PPS, there may be a risk that the resin insulating material separates from the spool, and if this occurs, there may be a risk that the insulation breakdown between the high-voltage side and the low-voltage side can be facilitated.
Namely, a slight gap existing between the resin insulating material and the spool may damage the insulation therebetween. Due to this, in the prior art, it was arranged for ignition coils to use, as a base resin for constituting a spool, a resin having a high adhesion to the resin insulating material so that, if unavoidable, there is formed, between the resin insulating material and the spool, as small a gap as possible.
As has been described heretofore, base resins for constituting the spool have conventionally been required to have the high electric insulation and high adhesion to the insulating resin. However, with the high adhesion, cracks are produced in the spool. In contrast, with the low adhesion, the spool and the resin insulating material are made to separate from each other easily.
The inventor of the invention studied the relationship between the combination of the adhesion of the base resin constituting the spool to the resin insulating material and the electric insulation of the base resin and the failure of insulation. As a result, the inventor determined that the failure of insulation can be prevented, without using the separation tape, by using as the base resin for the spool, a resin having a low adhesion to the resin insulating material and high electric insulation.
An ignition coil of the invention was completed based upon this knowledge. Consequently, an object of the invention is to provide an ignition coil having high electric insulation and which can be fabricated at reduced costs by obviating the necessity of a separation tape.
In addition, another object of the invention is to provide a method for fabricating the ignition coil of the invention relatively easily.
With a view to solving the problems, according to the invention, there is provided an ignition coil having a case, a rod-like core installed in the case, a cylindrical primary spool disposed substantially coaxially around an outer circumference of the core within the case, a primary coil comprising a wire wound around the primary spool, a cylindrical secondary spool disposed substantially coaxially around the outer circumference of the core within the case, a secondary coil comprising a wire wound around the secondary spool and a resin insulating material filled within the case, the ignition coil being characterized in that the spool of the primary and secondary spools which is disposed between the secondary coil and the core and/or which is disposed between the secondary coil and the primary coil comprises a base resin having an adhesive strength to the resin insulating material which is less than that provided by polybutylene terephthalate and an insulation breakdown voltage which exceeds that provided by polyphenylene sulfide.
In short, the ignition coil according to the invention is such that at least one of the primary and secondary spools is formed of the base resin having the adhesive strength which is less than that provided by PBT, as well as the insulation breakdown voltage which exceeds that provided by PPS.
Here, the adhesive strength to the resin insulating material is a parameter for evaluating the adhesion of the base resin to the resin insulating material. The higher the adhesive strength becomes, the higher the adhesive quality becomes. Note that the adhesive strength is measured using a measuring method shown in an embodiment which will be described later. In addition, the insulation breakdown voltage is a parameter for evaluating the electric insulation. The higher the insulation breakdown voltage becomes, the higher the electric insulation becomes. The insulation breakdown voltage is also measured using a measuring method shown in the embodiment which will be described later.
With the ignition coil according to the invention, the adhesive strength of the base resin constituting the spool to the resin insulating material is low. Due to this, there may be a risk that a separation is produced between the spool and the resin insulating material. However, even if the separation occurs, as the electric insulation of the base resin is high, there will be little chance that there is a risk that an insulation breakdown occurs between the high-voltage side and the low-voltage side.
In short, the ignition coil according to the invention is such that the spool and the resin insulating material are caused to separate from each other, as if it were intentional, by molding the spool itself from the base resin having the low adhesive strength to the resin insulating material to thereby restrain the production of cracks in the spool. Thus, with the ignition coil of the invention, even if the spool and the resin insulating material are caused to separate from each other, the insulation breakdown is prevented from occurring between the high-voltage side and the low-voltage side due to high electric insulation.
According to the ignition coil of the invention, a high electric insulation can be secured. In addition, according to the ignition coil of the invention, for example, a separation tape need not be wound around the spool nor does a rubber component need to be added into the base resin constituting the spool. Due to this, the construction of the ignition coil can be made simple and, therefore, the production costs can be reduced.
In addition, with a view to solving the problems, according to the invention, there is provided an ignition coil having a case, a rod-like core installed in the case, a cylindrical primary spool disposed substantially coaxially around an outer circumference of the core within the case, a primary coil comprising a wire wound around the primary spool, a cylindrical secondary spool disposed substantially coaxially around the outer circumference of the core within the case, a secondary coil comprising a wire wound around the secondary spool and a resin insulating material filled within the case, the ignition coil being characterized in that the spool of the primary and secondary spools which is disposed between the secondary coil and the core and/or which is disposed between the secondary coil and the primary coil comprises a base resin having an adhesive strength to the resin insulating material which is less than that provided by polyethylene terephthalate and an insulation breakdown voltage which exceeds that provided by polyphenylene sulfide.
In short, the ignition coil according to the invention is such that at least one of the primary and secondary spools is formed of the base resin having the adhesive strength which is less than that provided by PET, as well as the insulation breakdown voltage which exceeds that provided by PPS.
With the ignition coil according to the invention, the adhesive strength of the base resin constituting the spool to the resin insulating material is low. Due to this, there may be a risk that separation is produced between the spool and the resin insulating material. However, even if the separation occurs, as the electric insulation of the base resin is high, there will be little chance of a risk that an insulation breakdown occurs between the high-voltage side and the low-voltage side.
In short, the ignition coil according to the invention is such that the spool and the resin insulating material are caused to separate from each other, as if it were intentional, by molding the spool itself from the base resin having the low adhesive strength to the resin insulating material to thereby restrain the production of cracks in the spool. Thus, with the ignition coil of the invention, even if the spool and the resin insulating material are caused to separate from each other, the insulation breakdown is prevented from occurring between the high-voltage side and the low-voltage side due to high electric insulation.
According to the ignition coil of the invention, high electric insulation can be secured. In addition, according to the ignition coil of the invention, for example, separation tape need not be wound around the spool nor does a rubber component need to be added into the base resin constituting the spool. Due to this, the construction of the ignition coil can be made simple, and therefore, the production costs can be reduced.
Preferably, the base resin is a syndiotactic polystyrene. The adhesive strength of the syndiotactic polystyrene is less than that provided by PBT and hence is very low. Additionally, the insulation breakdown voltage of the syndiotactic polystyrene exceeds that provided by PPS and hence is very high. Due to this, in a case where the spool is formed of the syndiotactic polystyrene, even if the spool separates from the resin insulating material, there is little chance of a risk that the insulation between the high-voltage side and the low-voltage side is broken down. In addition, the syndiotactic polystyrene provides a high fluidity when it is molten during injection molding. From this viewpoint, the syndiotactic polystyrene is preferable as a base resin for constituting the spool.
The ignition coil according to the invention preferably embodies a stick-type ignition coil which is installed in a plug hole in a cylinder.
The ignition coil according to the invention can maintain high electric insulation for a long time even in a severe thermal-cycling environment. Additionally, according to the ignition coil of the invention, a separation tape need not be wound around the spool. This can facilitate making the ignition coil smaller in outside diameter. Consequently, the ignition coil according to the invention is suitable for a stick-type ignition coil that is subjected to severe changes in temperature and which needs to be made smaller in outside diameter.
In addition, with a view to solving the problems, according to the invention, there is provided an ignition coil having a case, a rod-like core installed in the case, a cylindrical primary spool disposed substantially coaxially around an outer circumference of the core within the case, a primary coil comprising a wire wound around the primary spool, a cylindrical secondary spool disposed substantially coaxially around the outer circumference of the core within the case, a secondary coil comprising a wire wound around the secondary spool and a resin insulating material filled within the case, the ignition coil being characterized in that the primary and secondary spools which are disposed between the secondary coil and the core and disposed between said secondary coil and said primary coil comprise a base resin which can hold electric insulation even if a high voltage is produced in the secondary coil in association with the generation of a separation between the resin insulating material and the spool.
With the base resin of the ignition coil according to the invention, even if there occurs a separation between the resin insulating material and the spool, the insulation between the secondary coil side and the primary coil side can be ensured. In other words, even if there occurs a separation, there is little chance that the insulation between the high-voltage side and the low-voltage side is broken down.
Preferably, the syndiotactic polystyrene is an improved syndiotactic polystyrene whose coefficient of linear expansion can be adjusted, and the coefficient of linear expansion of an end portion of the spool comprising the improved syndiotactic polystyrene is 135% or less, assuming that the coefficient of linear expansion of the resin insulating material is 100%.
The reason why the coefficient of linear expansion is set equal to or less than 135% is because, as will be described later, if the coefficient of linear expansion of the end portion of the spool exceeds 135%, the expansion of the end portion becomes much larger than the expansion of the resin insulating material. It is also because of a concern that there may be caused a defect in the resin insulating material and/or the spool.
Preferably, the improved syndiotactic polystyrene is formed by adding reinforced fibers into a syndiotactic polystyrene, and the reinforced fibers are oriented at random or circumferentially at the end portion of the spool.
When the reinforced fibers are dispersed at random or circumferentially, the coefficient of linear expansion of the end portion of the spool can be reduced. This makes it possible to reduce the difference in expansion between the resin insulating material and the end portion. Consequently, according to the construction, the risk is reduced that a defect is caused in the resin insulating material and/or the spool.
Preferably, the reinforced fibers are glass fibers and the resin insulating material is an epoxy resin. If the combination of the reinforced fibers and the resin insulating material is limited to the aforesaid combination, it is ensured that the difference in expansion between the resin insulating material and the end portion can be reduced.
Additionally, with a view to solving the problems, according to the invention, there is provided a method for fabricating an ignition coil having a spool comprising a winding portion around which a wire is wound and end portions disposed at longitudinal ends of the winding portion, the method comprising a spool material preparing process for preparing a spool material by adding reinforced fibers into a molten resin, a spool member molding process for injecting the spool material into a cavity in a mold from a gate disposed at a position which confronts an end portion molding part of the cavity, cooling the spool material so injected so that the spool material sets in the cavity, and molding a spool member in which the reinforced fibers are oriented at random or circumferentially at the end portion, and a gate cutting process for cutting a portion of the spool member which corresponds to the gate.
In short, the ignition coil fabricating method according to the invention is such as to have the spool material preparing process, the spool member molding process and the gate cutting process. Among these processes, in the spool material preparing process, the reinforced fibers are added to and dispersed in the molten resin. Then, the spool material constituting the raw material of the spool is prepared. In addition, in the spool member molding process, the reinforced fibers constitute the spool member in which the reinforced fibers are oriented at random or circumferentially at the end portions thereof. Furthermore, in the gate cutting process, the gate corresponding portions which are linked with the end portion of the spool are cut. The spool so obtained is then disposed within the case together with the other members, and the resin insulating material is then filled in the case, whereby the ignition coil of the invention is completed. According to the fabrication method of the invention, the ignition coil having the spool in which the reinforced fibers are oriented can be fabricated relatively easily.
Preferably, the gate is a ring gate or a film gate. According to the construction, the reinforce fibers can be oriented more easily. Consequently, the ignition coil of the invention can be fabricated more easily. However, the ignition coil of the invention in which the reinforced fibers are oriented can be fabricated not only by the aforesaid fabrication method according to the invention but also by other known fabrication methods.
Additionally, with a view to solving the problems, according to the invention, there is provided an ignition coil having a case, a rod-like core installed in the case, a cylindrical primary spool disposed substantially coaxially around an outer circumference of the core within the case and having a winding portion around which a winding is wound, a cylindrical secondary spool disposed substantially coaxially around the outer circumference of the core within the case and having a winding portion around which a winding is wound, and a resin insulating material filled and set within the case, the ignition coil being characterized in that at least one of the primary and secondary spools is an SPS spool comprising a syndiotactic polystyrene as a base resin.
In short, in the ignition coil according to the invention, at least one of the primary and secondary spools is an SPS spool. As has been described above, the adhesive strength of the syndiotactic polystyrene to the resin insulating material is very low. Consequently, according to the ignition coil of the invention, the thermal stress attributed to the coefficient of linear expansion can be relaxed. In addition, if one of the spools is made to be a SPS spool, the thermal stress of the SPS spool can be relaxed, whereby the thermal stress of the other spool which is attributed to the thermal stress of the one spool can also be relaxed. Furthermore, the electric insulation of the syndiotactic polystyrene is very high. Consequently, according to the ignition coil of the invention, even if the SPS spool separates from the resin insulating material, the risk is low that the insulation between the high-voltage side and the low-voltage side is broken down. Thus, according to the ignition coil of the invention, the high thermal stress relaxation and high electric insulation can be provided at the same time.
Preferably, the primary spool is the SPS spool. The voltage of the winding wound around the primary spool is lower than the voltage of the winding wound around the secondary spool. Due to this, by using the SPS spool for the primary spool rather than the secondary spool the risk can be reduced that a failure such as an insulation breakdown is caused, for example, in the spool situated adjacent to the separation space by the separation of the SPS spool from the resin insulating material. Consequently, the ignition coil constructed according to the invention can provide a high reliability against a failure such as the insulation breakdown.
Preferably, the adhesive strength of the base resin to the resin insulating material is less than 15 MPa.
Below is a reason for setting the adhesive strength less than 15 MPa. An FEM analysis (an analyzing software, Design Space available from Cybernet System Co., Ltd.) was carried out as to a thermal stress (tensile stress) which acts on the spool by the contraction of the resin insulating material when there occurs no separation between the spool and the resin insulating material. The result of the analysis showed that a tensile force that acted on the spool was 24 MPa.
Consequently, in case the adhesive strength is set less than 24 MPa, the SPS spool can be separated from the resin insulating material. However, variations in dimensions of the respective members constituting the ignition coil and variations and changes in material properties of the respective members have to be taken into consideration. Even with the adhesive strength being less than 24 MPa, there may be incurred the risk that a defect such as a crack is generated in the SPS spool depending upon the variations. Furthermore, there may be incurred the risk that a defect is caused in the other spool. For these reasons, the adhesive strength of the base resin to the resin insulating material was set less than 15 MPa to secure a safety margin relative to 24 MPa.
Preferably, a gap is formed between the winding portion of the SPS spool and the resin insulating material that has penetrated and set between turns of the winding wound around the winding portion, and wherein the gap is formed in such a manner as to extend over 70% or more of the surface area of the winding portion. Assuming that the total surface area of the winding portion is 100%, the gap is formed to extend over 70% or more of the total surface area. The reason why the gap is formed to extend over 70% or more of the surface area of the winding portion is because, if the gap extends over less than 70% of the surface area of the winding portion, a difference in linear expansion coefficients of the respective members constituting the ignition coil makes it easier for the thermal stress to be transmitted to the SPS spool. Then, there may be incurred the risk that a defect such as a crack is generated in the SPS spool, as well as the other spool. Note that when used in this invention, the winding portion denotes a portion of the spool which has a coil on the outer circumferential surface thereof, as shown in FIG. 4 which will be described later.
Preferably, the gap is formed in such a manner as to extend over 90% or more of the surface area of the winding portion. According to the construction, the risk is diminished that a defect such as a crack is generated in the spool, as well as the other spool, even if the vehicle is used in a severe thermal environment such as is seen when the vehicle is used in a severely cold or hot area, the vehicle is driven to climb up slopes, the vehicle is driven with the accelerator pedal being fully depressed such in racing, or the vehicle is used for a long period of time. Namely, the ignition coil according to the invention has a high durability relative to the thermal environment.
Preferably, a gap is formed between the winding portion of the SPS spool and the resin insulating material that has penetrated and set between turns of the winding wound around the winding portion, and wherein the radial width of the gap is 0.01 mm or greater. The reason why the radial width of the gap is made 0.01 mm or greater is because with the radial width of the gap being less than 0.01 mm, a gap is substantially not formed, and consequently, the thermal stress is easily transmitted to the spool, as well as to the other spool.
Preferably, the radial width of the gap is less than 0.3 mm. Below is a reason for setting the radial width of the gap less than 0.3 mm. Namely, in a case where the SPS spool is disposed radially outwardly of the other spool, the gap is interposed between a coil (for example, the primary coil) constituted by a winding wound around the SPS spool and a coil (for example, the secondary coil) constituted by a winding wound around the other spool. Due to this, if the radial width of the gap is large, the insulation distance between the primary and secondary coils becomes shorter substantially to such an extent that the radial width is increased. The radial width of the gap is set less than 0.3 mm from this reason.
Preferably, the radial width of the gap is 0.01 mm or greater and the gap is formed in such a manner as to extend over 70% or more of the surface area of the winding portion. According to the construction, the thermal stress transmitted from the resin insulating material to the SPS spool can be relaxed in a more ensured fashion.
Preferably, the radial width of the gap is 0.01 mm or greater and the gap is formed in such a manner as to extend over 90% or more of the surface area of the winding portion. According to the construction, the thermal stress transmitted from the resin insulating material to the SPS spool and the thermal stress transmitted to the other spool can be relaxed in a more ensured fashion.
Preferably, the insulation breakdown voltage of the base resin is 15 kV/mm or greater when measured using a measuring method of JIS (Japanese Industry Standard) K 6911. According to the construction, the insulation breakdown voltage of the syndiotactic polystyrene is set to 15 kV/mm or greater.
Below is the reason why the insulation breakdown voltage is set 15 kV/mm or greater. An FEM analysis (an analyzing software, Design Space available from Cybernet System Co., Ltd.) was carried out as to a field strength that is generated in the spool. The result of the analysis showed that a field strength generated in the spool was 14.5 kV.
Consequently, in case the insulation breakdown voltage is set 14.5 kV or greater, the insulation can be ensured. However, variations in dimensions of the respective members constituting the ignition coil and variations and changes in material properties of the respective members have to be taken into consideration. From these reasons, the insulation breakdown voltage of the base resin was set 15 kV or greater in order to secure a certain safety margin relative to 14.5 kV.
With the insulation breakdown voltage being 15 vK or greater, the outside diameter of the ignition coil can be reduced with no insulation breakdown being generated in the base resin even if the ignition coil is used in an environment where a relatively high voltage is applied to the base resin. For example, an ignition coil can be obtained which can apply a high voltage of 30 kV to a spark plug when inserted in a plug hole.
Preferably, the case is formed from a high-adhesion resin having a higher adhesion to the resin insulating material than to the base resin. The high-adhesion resin forming the case has the higher adhesion to the resin insulating material than to the base resin. Consequently, the resin insulating material is drawn toward interior surfaces of the case within the case. Due to this, according to the construction, the separation of the resin insulating material from the SPS spool can be facilitated further. Consequently, a gap can be easily formed between the resin insulating material and the SPS spool.
In addition, with a view to solving the problems, according to the invention, there is provided an ignition coil having a case, a rod-like core installed in said case, a cylindrical primary spool disposed substantially coaxially around an outer circumference of the core within the case and having a winding portion around which a winding is wound, a cylindrical secondary spool disposed substantially coaxially around the outer circumference of the core within the case and having a winding portion around which a winding is wound, and a resin insulating material filled and set within said case, the ignition coil being characterized in that a gap is formed between the winding portion possessed by at least one of the primary and secondary spools and the resin insulating material that has penetrated and set between turns of the winding wound around the winding portion after the resin insulating material has set.
In the ignition coil according to the invention, the gap is formed between the winding portion possessed by at least one of the primary and secondary spools and the resin insulating material that has penetrated and set between turns of the winding wound around the winding portion. According to the ignition coil of the invention, a thermal stress applied to the spool from the thermosetting resin can be cut off by the gap. This can restrain the occurrence of a risk that a defect such as a crack is generated in the spool.
Preferably, the spool situated adjacent to the gap is the primary spool. The voltage of the winding wound around the primary spool is lower than that of the winding wound around the secondary spool. Due to this, by disposing the primary spool rather than the secondary spool adjacent to the gap, for example, a risk that a defect such as an insulation breakdown is caused in the spool disposed adjacent to the gap can be reduced by the gap. Consequently, the ignition coil according to the invention is highly reliable against a defect such as insulation breakdown.
Preferably, a base resin composing the spool situated adjacent to the gap is a syndiotactic polystyrene. As has been described before, the insulation breakdown voltage of syndiotactic polystyrene is very high. Consequently, according to the ignition coil constructed as has been described above, irrespective of the formation of the gap, the risk is low that the insulation between the high-voltage side and the low-voltage side is broken down. Therefore, the ignition coil constructed according to the invention can provide a high thermal stress relaxing quality, as well as a high electric insulation quality.
Preferably, the gap is formed in such a manner as to extend over 70% or more of the surface area of the winding portion. The reason why the gap is formed so as to extend over 70% or more of the surface area of the winding portion is, as has been described above, because in case the gap is formed so as to extend over less than 70% of the surface area of the winding portion, a difference in linear thermal expansion coefficient between the respective members constituting the ignition coil facilitates the transmission of the thermal stress to the SPS spool. In addition, this is because there may be incurred the risk that a defect such as a crack is generated in the SPS spool, as well as the other spool.
Preferably, the gap is formed in such a manner as to extend over 90% or more of the surface area of the winding portion. As has been described above, according to the construction, even if the vehicle is used in the thermally severe environment, a risk that a defect such as a crack is generated in the spool can be maintained low. Namely, the ignition coil constructed according to the invention is highly durable against a thermal environment.
Preferably, the radial width of the gap is 0.01 mm or greater. As has been described above, the reason why the radial width of the gap is made 0.01 mm or greater is because, with the radial width of the gap being less than 0.01 mm, a gap is substantially not formed, this facilitating the transmission of the thermal stress to the spool.
Preferably, the radial width of said gap is less than 0.3 mm. Below is the reason why the radial width of the gap is made less than 0.3 mm. Namely, as has been described above, in a case where the spool disposed adjacent to the gap is disposed radially outwardly of the other spool, in case the radial width of the gap is large, the insulation distance between the primary and secondary coils becomes substantially shorter to such an extent that the radial width is increased.
Preferably, the radial width of the gap is 0.01 mm or greater and the gap is formed in such a manner as to extend over 70% or more of the surface area of the winding portion. This construction ensures further that the thermal stress transmitted from the resin insulating material to the spool disposed adjacent to the gap can be relaxed.
Preferably, the radial width of the gap is 0.01 mm or greater and the gap is formed in such a manner as to extend over 90% or more of the surface area of the winding portion. This construction further ensures that the thermal stress transmitted from the resin insulating material to the spool disposed adjacent to the gap can be relaxed.
Preferably, the insulation breakdown voltage of the base resin composing the spool situated adjacent to the gap is 15 kV/mm or greater when measured using the measuring method of JIS K 6911. According to the construction, the insulation breakdown voltage of the base resin is set to 15 kV/mm or greater.
As has been described above, the reason why the insulation breakdown voltage is set 15 kV/mm or greater is because the safety margin is secured relative to the field strength of 14.5 kV obtained the FEM analysis. In a case where the insulation breakdown voltage is 15 kV or greater, the outside diameter of the ignition coil can be reduced with no insulation breakdown being generated in the base resin even if the ignition coil is used in an environment where a relatively high voltage is applied to the base resin. For example, an ignition coil can be obtained which can apply a high voltage of 30 kV to a spark plug when inserted in a plug hole.
Preferably, the insulation breakdown voltage of the base resin composing the spool situated adjacent to the gap is 15 kV/mm or greater when measured using a measuring method for actually measuring the spool itself. The method for measuring an insulation breakdown voltage by the aforesaid JIS K 6911 is a method for measuring an insulation breakdown voltage by applying a voltage to a test piece. In contrast, the method for measuring an insulation breakdown voltage according to the invention is a method for directly measuring the insulation breakdown voltage of the spool itself.
A conceptual measuring method constructed according to the invention is shown in FIG. 10. A rod-like electrode 501 which is grounded is inserted in a cylindrical spool 500. In addition, another electrode 502 is disposed on an outer circumferential surface of the spool 500. Namely, a cylindrical wall of the spool 500 is held by the two electrodes 501, 502. The voltage applied to the two electrodes 501, 502 is gradually increased, and a voltage at which an electrical communication is established between the electrodes 501, 502 is the insulation breakdown voltage of the invention. According to the construction of the invention, the insulation breakdown voltage can easily be measured without preparing a test piece separately. Here, the reason why the insulation breakdown voltage is set 15 kV/mm or greater is because, as has been described above, the safety margin is secured relative to the field strength of 14.5 kV which was obtained by the FEM analysis.
Preferably, the adhesive strength of the base resin composing the spool situated adjacent to the gap to the resin insulating material is less than 15 MPa. Here, the reason why the adhesive strength is set less than 15 MPa is because, as has been described above, the safety margin is secured relative to the tensile stress of 24 MPa which was obtained by the FEM analysis.
Furthermore, with a view to solving the problems, according to the invention, there is provided a method for fabricating an ignition coil having a case, a rod-like core disposed in said case, a cylindrical inner spool disposed substantially coaxially around an outer circumference of the core within the case and having a winding portion around which a winding is wound, a cylindrical outer spool disposed substantially coaxially around the outer circumference of the core within the case, possessing a winding portion around which a winding is wound and having an outer circumferential surface having a lower adhesion to a resin insulating material than to an inner circumferential surface of the case, and the resin insulating material filled and set within the case, the method comprising an insulating material filling process for filling the resin which is something like a liquid into the case in which the respective members are disposed, an insulating material gelling process for gelling the resin insulating material so filled at a high temperature, and an insulating material cooling process for cooling the resin insulating material so gelled together with the case and the outer spool.
In other words, the ignition coil fabricating method according to the invention is such as to have the insulation material filling process, the insulation material gelling process and the insulation material cooling process. Among the processes, in the insulation material filling process, the members such as the primary spool and the secondary spool are first disposed within the case, and next, the liquid-like resin insulation material is filled within the case. In the insulation material gelling process, the resin insulation material is held for a predetermined period of time at the setting temperature so that the resin insulation material is gelled. In the insulation material cooling process, the thermosetting resin in which a setting reaction is completed is cooled. The resin insulation material is separated from the outer circumferential surface of the outer spool during the cooling of the thermosetting resin because the adhesion between the outer circumferential surface of the outer spool and the resin insulation material is lower than the adhesion between the inner circumferential surface of the case and the resin insulation material. When the thermosetting resin is filled through the processes the gap is formed between the winding portion possessed by at least one of the primary and secondary spools and the resin insulation material that penetrates between turns of the winding wound around the winding portion for setting thereat. Namely, the ignition coil according to the invention can be fabricated by the ignition coil fabricating method according to the invention.
In addition, the fabricating method according to the invention is such as to form the gap by making use of the total contraction of the resin insulation material. A typical volume change happening during the setting process of the thermosetting resin is shown in FIG. 11. In the figure, the axis of abscissa represents temperatures. In the figure the axis of ordinates represents volumes. As shown in the figure, firstly, the volume of the liquid-like thermosetting resin increases due to the simple thermal expansion of the liquid happening as it is heated from point A to point B (to the setting temperature). Next, from point B to point C, the thermosetting resin is held at the thermosetting temperature for the predetermined period of time. As this happens, the thermosetting resin is transformed from a liquid to a gelled state through the thermal reaction. Then, the volume of the thermosetting resin decreases. Finally, from point C to point D, the thermosetting resin in which the thermal reaction is completed is cooled down to the room temperature. As this occurs, the volume of the thermosetting resin decreases further. As a result, the volume at point D becomes smaller than the volume at point A. This is referred to as the total contraction.
According to the fabricating method of the invention, the ignition coil of the invention can be fabricated relatively easily by making use of the total contraction. The ignition coil of the invention can, however, be fabricated by not only the fabricating method of the invention but also known fabricating methods.
The invention can be understood more sufficiently from the following description of preferred embodiments of the invention while referring to the accompanying drawings.